The present invention relates to non-aqueous electrolyte secondary batteries using non-aqueous electrolyte with lithium ion conductivity where material capable of incorporating and releasing lithium is used as a negative active material, and in particular, it relates to novel negative active materials capable of proposing improved high-reliable secondary batteries with a longer service life having a satisfactory charge and discharge characteristic of a high voltage and high energy density.
Non-aqueous electrolyte batteries using lithium as a negative active material have various advantages including high reliability ranging long periods due to smaller amounts of self-discharge in addition to a high voltage and higher energy density, therefore they have widely been used as primary batteries of power supplies for memory backup, cameras and the like. However in recent years, following the remarkable development of portable type electronics equipment and devices, communication equipment and devices and so forth, various kinds of equipment and devices have been realized requiring larger current outputs for batteries as a power supply. It is therefore strongly desired to produce high energy density secondary batteries capable of recharging and redischarging from the view point of economics, compact size, and light-weight of the devices. For this reason, the research and development for more satisfactory non-aqueous electrolyte secondary batteries have been considerably promoted, a part of which is now in practical use, however unsatisfactory characteristics still remain in energy density, charge and discharge cycle service life, and reliability.
Conventionally, as a positive active material constituting a positive electrode of the secondary battery of this kind there have been found three kinds of types depending on charge and discharge reaction patterns. Namely, in the first type, only lithium ions (cation) are input into and output from between layers of the crystal, lattice positions or gaps among lattices of the crystal by means of intercalation and deintercalation reactions and the like as is the case of metal chalcogenide such as TiS.sub.2, MoS.sub.2, NbSe.sub.3 and the like, and metal oxide such as MnO.sub.2, MoO.sub.3, V.sub.2 O.sub.5, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, Li.sub.x Mn.sub.2 O.sub.4, and the like. The second type is a type in which mainly only anion is stably input or output by the doping or undoping reactions as in the case of conductive polymers such as polyaniline, polypyrrole, polyparaphenylene and the like. The third type is a type in which both lithium cation and anion can be input and output as in the case of layer-like structure graphite compounds and conductive polymers such as polyacene and the like (intercalation, deintercalation, or dope, undope or the like).
On the other hand, in the negative active material of the battery of this kind, since the use of metal lithium in a simple substance gives the basest electrode potential, it is preferable that the battery combined with the positive electrode using positive active material as described above has the highest voltage and the highest energy density. However, the problem arises in considerable deterioration with charge and discharge and results in a shorter cyclic life because of the generation of dendrite or passive state compounds on the negative electrode due to the charge and discharge. In order to solve this problem, for the negative active material, various possible materials capable of incorporating and releasing lithium ions are proposed; namely, (1) alloy of lithium with other metals such as Al, Zn, Sn, Pb, Bi and Cd; (2) intercalation compounds or insertion compounds in which lithium ions are incorporated into the crystal structure of inorganic compounds such as WO.sub.2, MoO.sub.2, Fe.sub.2 O.sub.3, and TiS.sub.2, graphite, and carbonaceous materials obtained by baking organic materials; (3) conductive polymers such as polyacene, polyacethylene and the like in which lithium ions are doped.
However, in general, in case where, as a negative active material, the negative electrode using materials capable of incorporating and releasing lithium ions (other than metal lithium as described above) is combined with the positive electrode using the positive active material described above to constitute a battery, an electrode potential of the negative active material is nobler than an electrode potential of metal lithium, and the drawback therefore arises in that an operating voltage of the battery is lowered than when using metal lithium in the simple substance as a negative active material. For example, the operating voltage is lowered by 0.2 to 0.8 V when using alloys of lithium with Al, Zn, Pb, Sn, Bi, Cd or the like, by 0 to 1 V when using lithium intercalation compound of carbon, and by 0.5 to 1.5 V when using lithium ion insertion compound such as MoO.sub.2 or WO.sub.2.
Since elements other than lithium are involved as negative electrode constituent elements, the capacity and energy density per volume and weight are considerably lowered.
Further, in case (1) where the alloys of lithium are used with the other metals, a problem occurs because the utilization-efficiency of lithium is low during charge and discharge and repeating charge and discharge causes cracks or breaks in the electrode which results in a shorter cyclic life. It (2) where the battery uses the lithium intercalation compound or insertion compound, deteriorations such as decay of the crystal structure and generation of irreversible substances arise in the case of excess charge and excess discharge, and further there is a drawback of a lower output voltage of the battery because of the higher (nobler) electrode-potential. In case (3), where the conductive polymer is used, the problem is that the charge and discharge capacity, in particular, the charge and discharge capacity per unit volume, is small.
For these reasons, to obtain a secondary battery with a long cyclic service life having a graded charge and discharge characteristic with a high voltage and a high energy density, there is required a negative active material having a larger effective charge and discharge capacity, that is, a larger amount of reversible incorporation and release of lithium ions with a lower (baser) electrode potential for lithium but without deteriorations such as decay of the crystal structure and generation of irreversible substances and the like due to the incorporation and release of the lithium ions during charging and discharging.